1. Field of the Invention
This invention relates to a method for mounting a magnetic plate to a disc substrate. More particularly, it relates to a method for mounting a magnetic plate to the substrate of an optical disc or a magneto-optical disc.
2. Description of the Related Art
There has hitherto been proposed a disc for recording information signals, such as an optical disc or a magneto-optical disc. Since this type of disc allows for high-density recording of information signals, there has been proposed a disc of an extremely small size, such as an optical disc or a magneto-optical disc which is approximately 64 mm in diameter and which allows for recording of music signals continuing for about 74 minutes.
The magneto-optical disc, which is of a small diameter and yet permits high density recording of information signals, is loaded on a driving unit so as to be rotated at a constant linear velocity. As the disc is rotated, a light beam is radiated from an optical pickup device onto fine recording tracks formed on a signal recording layer on a major surface of the disc. Simultaneously, an external magnetic field, modulated in accordance with recording information signals, is applied to the disc from an external magnetic field generating unit, such as a magnetic head, for recording desired information signals on the disc.
For correctly radiating the light beam to the fine recording track during high-speed rotation of the disc, it is necessary for the magneto-optical disc to be loaded in position on the driving unit such that the center of rotation of the disc is coincident with the center of rotation of the disc table. To this end, a disc loading system has been proposed in which a magnetic metal plate is provided on the side of the magneto-optical disc so as to be attracted by a magnet provided on the disc table for chucking the magneto-optical disc with respect to the disc table. This system is proposed, for example, in U.S. Pat. Nos. 4,926,410, 4,829,510, and 4,787,009.
The magneto-optical disc, employed in a magnetic chucking system, is provided with a disc substrate 51 molded of a synthetic resin, such as a transparent polycarbonate resin, in the form of a disk, as shown in FIG. 1. On one major surface 5 la of the disc substrate 51, there is formed a recording layer on which information signals are to be recorded. The major surface 51b of the disc substrate 51, opposite to the major surface 51a having the recording layer, is designed as a write/read surface for information signals. A light beam is radiated to the signal recording layer from the write/read surface for recording and/or reproducing desired information signals.
The mid part of the disc substrate 51 has a centering aperture 52 engaged by a centering member provided on the driving unit. A magnetic metal plate 53 in the form of a disk is provided at the center of the major surface 51a of the disc substrate 51 for closing the centering aperture 52. The magnetic plate 53 is disposed so as to be accommodated within a housing recess 54. The housing recess encircles the centering aperture 52 on the major surface 51a of the disc substrate 51. A number of protrusions 55 directed to the inner side of the housing recess 54 are formed by ultrasonic machining of the rim of the recess 54. The magnetic plate 53, placed within the recess 54, is positionally maintained by these protrusions 55 with respect to the disc substrate 51.
The magnetic plate 53 is mounted on the disc substrate 51 in the following manner. The disc substrate 51 is first set in position on a disc support base 56, as shown in FIG. 1. The disc support base 56 has a disc rest 57 for accommodating a protuberance 59. The protuberance 59 delimits the outer periphery of the recess 54 of the disc substrate 51 while positioning the disc substrate 51. Specifically, the disc substrate 51 is positioned by having the protrusion 59 accommodated within the disc rest 57.
As shown in FIG. 1, a horn 58 is placed in abutment with the rim part of the recess 54 of the disc substrate 51 set on the disc rest 57. To this horn 58 are transmitted ultrasonic vibrations from an ultrasonic oscillator, not shown. The ultrasonic vibrations are propagated in a direction parallel to the major surface 51a of the disc substrate 51, that is, they are horizontally directed ultrasonic vibrations.
The result is that the portion of the synthetic material in the region of the recess 54 in contact with the horn 58 is melted due to the heat of friction so that the melted synthetic material is caused to flow inwardly of the rim to form the protrusions 55. This causes the magnetic plate 53 to be loosely accommodated and held in the recess 54. It is noted that the protrusions 55 are formed at, e.g., four positions around the recess 54 to prevent the magnetic plate 53 from being disengaged from the disc substrate 51.
However, since the propagation of ultrasonic vibrations along the thickness of the disc substrate 51 is limited, only two protrusions 55 at most may be formed by each ultrasonic forming operation, despite the fact that the thickness of the protrusions 55 designed to prevent the magnetic plate from becoming detached may be controlled with high accuracy. The result is lower productivity due to increased tack time and insufficient strength of the protrusions 55.
With this consideration, attempts have been made to apply a columnar horn 58 perpendicular to the major surface 51a of the disc substrate 51, and to apply vertically directed vibrations to the horn 58, as shown in FIG. 2.
With the method of applying vertically directed ultrasonic vibrations, an annular protrusion 55 may be formed on the entire rim of the recess 54. This is achieved by contacting the horn 58 with the disc substrate 51 and applying longitudinal ultrasonic vibrations thereto. This method is directed to improving productivity with respect to tack time.
However, since the vibrations are applied perpendicularly to the disc substrate 51, it is extremely difficult to control the thickness T.sub.1 of the protrusion 55, as shown in FIG. 3. The result is that the magnetic plate 53 tends to be stationary with respect to the disc substrate 51. In other words, it is difficult to provide a clearance between the magnetic plate 53 and the protrusion 55. If the magnetic plate 53 is stationary with respect to the disc substrate 51, then disc substrate 51 tends to be deformed due to the difference between the thermal expansion coefficients of the magnetic metal plate 53 and the disc substrate of synthetic material 51. The result is that birefringence is produced in the disc substrate 51, deteriorating its optical properties and causing data write/read problems.